1. Field of Invention
This invention relates to improvements in aircraft flight control systems, and, more particularly, to improvements in interchannel data transfer techniques to facilitate dependent redundant computation operations in each channel.
2. Description of the Prior Art
Digital implementation of aircraft flight control systems has been becoming of increased interest, since such digitally implemented systems are generally regarded as being more efficient and more versatile than their analog counterparts, especially in view of the programmable capabilities of many computers characteristically used with such digital systems.
Typical flight control systems use dual, or, often times, triple redundant data receiving, computing, and outputting channels or paths. In one prior art system, for instance, described by J. C. Hall in a technical report published May 23, 1975, entitled Digital Flight Control for Transport Aircraft: An Approach to Efficient Design, at 6-1 et seq., a set of triple sensors are employed, each independent from the other, to produce desired independent measurements of such parameters as air speed, radio altitude, rate of descent, and the like. The outputs from the triple sensors are each directed to a respective associated computation channel which processes the data in a desired manner. Additionally, the data derived from the sensors associated with the other computation channels is exchanged among the channels, and the exchanged information is additionally processed within each channel. Thus, for example, typically each channel will receive data inputs from each of the three selected sensor inputs to produce three independent and comparable processed information indications. It can be seen that if one of the sensors fails, that fact will be readily apparent in each of the processing channels, since each channel will be processing the same erroneous data. By the provision of an appropriate computer algorithm, the fact that erroneous data is being received and processed can be readily determined to eliminate that sensor's data from the data computation channels. Since there would remain two operative sensors, the overall system would remain operational, despite the sensor failure.
In addition, the computed information from each computation channel is circulated or exchanged among each of the other computation channels, in a manner like that of the sensor data exchange. Thus, each of the computation channels can monitor itself and the other channels, such that if one of the channels were to fail, the failing channel can be readily identified, and its computational results discarded so as not to affect the flight control provided by the system.
The triple computation channels, each presented with triple redundant computed information, each computed from triple redundant sensor information, can, if desired, then be individually voted upon within the individual computation channels to determine the most likely correct information for controlling each appropriate aircraft control surface. Triple output channels may also be provided, one for each respective computation channel, to provide votable output signals to manipulate a single control surface, such as aileron, rudder, trim tab, or the like.
It can be appreciated that by virtue of the triple redundant operation throughout the system, a truly "fail-operative" system can be achieved, that is, a system in which the failure of one sensor, or even one entire computation channel, or one output channel, will not affect in any way the overall operation of the system. This is essential to minimize the possibility of an uncontrolled "hardover" condition.
However, it can be seen that because of the triple redundant nature of the overall system, a large amount of data must be continuously rapidly circulated and processed. This is especially true if a large number of input sensors are anticipated, such as may typically be encountered in a flight control system on, for example, a wide body commercial jet. This problem is recognized in a technical report by J. C. Hall entitled Air Transport Flight Control: Progress from Analog-to-Digital Implementation, at 4-10.
In the prior art, this has been handled at two levels, as discussed by D. W. Mineck in his paper entitled Redundant Digital Flight Control: Cost Performance Trade Offs, Aug. 16, 1976. For instance, FIG. 15 of this paper discloses that, at the sensor input level, the sensor data is first digitized and applied to a shift register. The data is circulated within the shift register, and detected bit at a time for serial transmission to the other computation channels. After the data has been entirely circulated within the shift register, it is stored in a preassigned memory location in the respective associated memory unit of the computation channel. Concurrent with this data circulation, the data is serially transmitted to corresponding memory locations in the other computation channels. At the computed information level, the processed data is stored in a preassigned memory location in each respective data channel. When it is desired to transmit the data to the other computation channels, the data is moved into a specific memory section in this computation channel. Then, within the transmitting section, the data is recalled, formatted into a serial configuration, and transmitted to the other computation channels. Upon reception of the data in the other computation channels, the data is stored in the main memory associated therewith.
Because the data is transmitted in serial fashion, a large amount of time is required for its complete transfer, especially over a fairly large memory array of, for example, 64 words of 16 bits each. Typically, for example, the data when written in parallel into a memory can be achieved very fast, on the order of, for instance, one microsecond per word. Moving the serial data out, however, is comparatively slow, typically on the order of 100 microseconds per word. Thus, in systems in which large amounts of data are to be subjected to such interchannel exchange, many usable state-of-the-art computers are necessarily operated at or near their maximum data handling capabilities.